Yes. Same-sex marriage will deliver economic benefits

How on earth could same-sex marriage deliver 10 years' worth of economic benefits? And why on earth do 18 of Australia's leading economists expect it to?

The experts were surveyed this week by the Economic Society of Australia. Thirty answered this question: "Will changing the law to allow same-sex couples to marry generate net economic benefits for the nation as a whole over the next 10 years?"

Almost always whenever someone claims something will benefit the economy they are wrong. Look at a graph of GDP either side of the Sydney Olympics and you won't see anything other than a drop in GDP during the Games. Tourism flatlined then fell after the Games. It didn't start growing strongly again until 2004. Even the Olympic Stadium, which we were told would be a lasting legacy, is, according to the premier, so clapped out it ought to be torn down.

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Kirsty Albion and Kelly McKenzie, who plan to marry early next year, at the marriage equality plebiscite announcement.

KPMG, which wanted work associated with the Games, produced a much-hyped study for the NSW government saying the Olympics would boost economy growth by $7 billion. A decade later, an examination of what actually happened found it had clipped economic growth. Like countless expressways, stadiums and mega projects before and after it, it had cost more to create than it could ever produce in benefits.

One of the reasons the spruikers almost always get it wrong is that they add up the costs of the project (that's the easy bit) and then subtract them from the project's benefits. For sports events, those benefits include extra spending as people pour into Olympic Park or into Melbourne Park for the tennis. What the spruikers forget, often, is that the people who spend at big events would have spent something anyway, perhaps in their own suburb or at another sporting event or at the theatre. They forget to subtract the spending that won't be done in order that the spending at the big event can be done.

It's a trap for people expecting benefits from same-sex weddings. Professor Kevin Davis from Monash University put it this way in response to the Economic Society survey: "There may be more expenditure on weddings etc, but there is no obvious reason this would not be at the expense of other expenditures."

There can be a localised benefit in a country town. A really big wedding or special event can draw people into the town who never would have spent there. But the gain to that town will be a loss to the region or town from which those people have come.

So why are the experts so sure there will be benefits from permitting same-sex marriage?

Partly, because it's cheap. Passing a law costs nothing compared to building a stadium.

Here's how Lin Crase of the La Trobe University puts it: "Constraints that impinge on individuals' full participation in society necessarily reduce economic welfare. It follows that removal of those constraints should lead to some gains."

Constraints that impinge on individuals' full participation in society necessarily reduce economic welfare

Professor Lin Crase, La Trobe University

This would be true even for people in same sex relationships who decided not to take advantage of the opportunity to marry.

Professor Mardi Dungey of the University of Tasmania says that when we remove impediments to improving people's ability to satisfy their wants, with no material harm to others, we necessarily improve people's welfare.

And Curtin University's Professor Margaret Nowak identifies broader benefits: reduced health costs, especially in the area of mental health, reduced suicide rates among youth, and reduced discrimination in the workforce "resulting in more optimal allocation of workers".

For what it's worth, married couples also spend more. Dr Gigi Foster from the University of NSW says married heterosexual couples invest more in the kind of things that shacked-up couples don't. And she says something else.

Legalising same-sex marriage will allow politicians and the public to move on and focus on other things that could produce further economic benefits. There's a chance.